"Her novels are interesting."
Translation:Haar romans zijn interessant.
I think this is not an adjective but an adverb, because it says something about the verb. In this case: about the way her novels are. And adverbs don't conjugate. But please correct me if I'm wrong. (It could also be something with the predicate nominative but I have no idea how to explain that)
Also, how would you have made interessant plural? 'Cause adjectives get an -e in certain cases, not necessarily because something is plural, but never an -s.